One of the major research concerns at NIEHS is to understand the interactions between exogenous chemicals and biomolecules within the body. For a complete understanding of this interaction, the structures of the products of covalent interactions must be elucidated. Of special concern are the products of reactions between environmental carcinogens and DNA. The products of the in vitro reaction of benzo(a)pyrenediol-epoxide and oligonucleotides have been successfully analyzed by FAB/MS and FAB/MS/MS. Negative ion MS/MS gives sequence information identifying the bases which were modified while positive ion FAB identified the site of modification on the base. The products of the enzymatically mediated reaction of 3- nitrofluorene and DNA has been determined to be (after nuclease digestion) N-(deoxyguanosin-8-yl)3-aminofluoranthene. We are currently analyzing the 3-methyl-chrysenediol epoxide adduct with guanylic acid.